Measurement of the Proton's Weak Charge at the Qweak Experiment

TL;DR

The Qweak experiment measures the proton's weak charge via parity-violating electron scattering, providing a novel, precise test of the Standard Model and constraints on new physics at the TeV scale.

Contribution

First direct measurement of the proton's weak charge with high precision, testing Standard Model predictions and probing potential new physics.

Findings

Measured the proton's weak charge with 2.5% accuracy.

Provided constraints on physics beyond the Standard Model.

Validated experimental techniques for low-energy parity violation measurements.

Abstract

The Qweak experiment at Jefferson Laboratory measures the parity violating asymmetry of polarized electrons scattering from a proton target at very low momentum transfer. In the Standard Model, this asymmetry reveals the proton's coupling to the neutral vector current, the weak charge. This value, measured directly for the first time, will provide a precision test of the Standard Model and will constrain the possibility of relevant physics beyond the Standard Model. The planned precision will probe certain classes of new physics at the ~2 TeV scale. In order to challenge the precise predictions, the asymmetry will be measured with a 2.5 percent accuracy. To achieve such a precision, great care has to be taken on many aspects of the experiment. The very low momentum transfer reduces the hadronic effects to the asymmetry and must be determined to half of a percent accuracy. Beam stability is controlled and monitored constantly and background events are carefully studied.